There are a lot of cat owners who are concerned about their feline friend contracting heartworms, but many of the same owners don't know where heartworms come from or how cats become infected.  Grab your heartgard and get a pen and paper! It's time to learn about one of the most troublesome and dangerous pet parasites.
Regardless of if you have a dog or a cat, all heartworm infestations start with the same thing: a mosquito bite.  Heartworm infestation begins when a mosquito that's carrying heartworm larvae bites your cat.  The larvae are injected into the body through the bite, and then they migrate throughout the body and slowly develop.  It takes a while for the heartworms to migrate through the animal's subcutaneous tissue, but after around 3-4 months of being in the body they settle into the arteries and blood vessels of the lungs.
Heartworms take slightly longer to fully mature in cats than they do in dogs, most dog heartworms have reached maturity after 7 months and most cat heartworms take 8 months. After the worms reach maturity they start to mate.  Female heartworms release microfilariae into the host's blood stream, and the cycle begins all over again when a mosquito drinks blood from the cat and unknowingly ingests heartworm larva.
Typically cats are less susceptible to heartworms than dogs, and some cats are more naturally resistant to heartworms.  Only 20% of felines infected with heartworms will have new heartworm larva in their blood, some with strong immune systems are even able to spontaneously rid themselves of the infection.  Despite these facts it's still important for all cat owners to have their cats routinely tested for heartworms.  For more information about feline heartworms, visit the Heartworm Society's website.